Portable terminals, such as portable telephones, having a whip antenna or a built-in antenna, and, in addition to these antennas, mounted with an antenna element for wireless data communication with other electronic equipment are increasing.
Also, portable mobile electronic equipment, such as notebook personal computers, for wireless data communication are increasing, and those mounted with an antenna in the electronic equipment are also increasing.
As the above-mentioned antenna element, there is an antenna element having a helical conductor on a prism-like insulating substrate, of which both ends are terminals and one of the terminals serves as a feeder terminal (e.g., refer to Japanese Laid-open Patent 2001-326522). In this case, λ/4 type antenna is often used for reducing the mounting area (λ is wavelength of transmit-receive electric waves). FIG. 36 is a perspective view of an antenna element in the prior art. Substrate 104 is made from a square insulating material such as ceramic, and there are provided terminals 101 and 102 at the ends thereof. A signal source is connected to one of the terminals 101 and 102. Helical conductor 103 is formed by winding a copper wire or the like or trimming the conductive surface plated on the substrate 103. Since such antenna element can be very much reduced in size, it can be easily mounted inside or outside a portable terminal.
Here, the λ/4 type antenna operates with image current generated on a ground surface of a conductor existing in its vicinity. Therefore, it is most desirable to install the antenna element in a direction vertical to the ground surface.
FIG. 37 is a diagram of the relationship between the antenna element and the ground surface in the prior art, and shows the relationship between the λ/4 type antenna and an image current generated on the ground surface. The current is supplied to λ/4 type antenna 110 via feeder line 112. Current I flows in the λ/4 type antenna 110, and image current I′ flows in the ground surface 111. The image current I′ is generated in the direction of current I as shown in FIG. 37 and serves for increasing the gain of transmitting and receiving operation at the λ/4 type antenna 110. In this way, since the conductor is installed vertical to the ground surface 111, the λ/4 type antenna 110 is optimized in its transmitting and receiving operation.
However, when a conventional antenna element is mounted inside or outside a portable terminal, there may often exist various conductors in the vicinity of the antenna element. There are conductors such as a casing, other parts, and a substrate, which may sometimes exist almost in parallel with the antenna. Such conductors are often connected to the ground and become a ground surface.
Thus, when the antenna element is placed substantially parallel with metallic parts, image current flowing in a direction opposite to the current flowing in the antenna element is generated on the conductor being the ground surface. In case such an image current is generated, a force is generated to cancel the current flowing on the antenna element, causing the gain at the antenna element to be decreased. Naturally, even in a case of not being placed in substantially parallel fashion, if the antenna element is not installed substantially vertical to the ground surface, a force is similarly generated to decrease the gain due to image current. With the miniaturization and further integration of electronic equipment such as portable terminals, it has recently become more and more difficult to install an antenna element in the best conditions. Also, for installing the antenna element in best conditions, it is necessary to assure that there is a sufficient mounting zone and there arises a problem of increase in size of the electronic equipment.
Further, when a conductor exists in the vicinity of the antenna element, the directionality of the conductor affects the directionality of the antenna element and there is a problem of lowering of transmitting-receiving performance of the antenna element.